Along with the accelerated digitalization of household electric appliances and increase in CPU speed in recent years, there has increasingly been a public concern about problems of electromagnetic interference that adversely affect peripheral equipment and human bodies. In view of these problems, “Voluntary Control Council for Interference by Information Technology Equipment (VCCI)” has been established in Japan and there has increasingly been a tendency that the relevant industries voluntarily impose controls on the EMI problems to comply with regulations of VCCI. Examples of such voluntary control may include a technology of enclosing an electronic substrate or the like in an electric/electronic appliance by a shield box formed of a metallic (conductive) material to shield electromagnetic wave to suppress electromagnetic noise generated from the electronic substrate.
The shield box is adapted to shield against electromagnetic wave by reflecting the electromagnetic wave by a conductive material forming the shield box. The higher conductivity of the material forming the shield box results in the higher reflectance of the electromagnetic wave, thereby increasing the electromagnetic wave shielding properties. For this reason, it is important that the metallic sheet forming the shield box has high conductivity to ensure good electromagnetic wave shielding properties of a shield box.
Further, a shield box, which is generally manufactured by forming a metallic sheet, tends to have discontinuous portions (such as seams and junctions) therein and be susceptible to leakage and intrusion of electromagnetic wave through the discontinuous portions. In view of this, a shield box generally has a conductive gasket inserted in the discontinuous portions, to thereby prevent leakage and intrusion of electromagnetic wave.
In this regard, to further ensure shielding properties of the shield box, the shield box needs to be configured to allow desired current to pass through/across the entire shield box. However, the contact portion between the above-mentioned metallic body and the gasket is generally low in contact pressure, whereby the electrical continuity between the metallic body and the gasket (which will be simply referred to as “continuity” hereinafter) is inferior and thus an amount of current passing through the contact portion is relatively small. Therefore, it is important to ensure good continuity between the metallic body and the gasket, in addition to ensuring good conductivity of the metallic sheet itself constituting the shield box, in terms of further improving the performance of the shield box.
Meanwhile, electric and electronic equipments are used under various environments nowadays. Hence, a material constituting a shield box is required to be corrosion resistant, i.e., to exhibit good corrosion resistance even under severe usage environment. Further, a shield box may be spot-welded in the forming process, and thus required to have stable weldability to ensure high productivity.
Conventionally, there has been widely applied, to a steel sheet for use in household electric appliances, building materials, and automobiles, a zinc or zinc alloy coated steel sheet having undergone a chromate treatment for the purpose of improving corrosion resistance (white rust resistance, red rust resistance), the chromate treatment using a treatment solution containing, as a main component, chromic acid, dichromic acid or the salts thereof.
As described above, a metallic body (steel sheet) constituting a shield box is required to have relatively high conductivity and, in particular, exhibit good continuity with respect to a gasket. In this regard, a coating film formed on the steel sheet by chromate treatment can exhibit good rust resistance even if the coating film is relatively thin, which provides good weldability, despite the coating film being inferior in conductivity than the base steel sheet. That is, a surface-treated steel sheet subjected to chromate treatment can attain conductivity equivalent to a (non-surface treated) steel sheet by making a less conductive coating film thereof as thin as possible, to sufficiently ensure good continuity of the shield box with respect to the gasket. As a result, good rust resistance and weldability can be attained along with the good electromagnetic wave shielding properties. However, in light of the recent global environmental problems, there is an increasing demand to adopt a nonpolluting surface-treated steel sheet without recourse to chromate treatment, which is so-called a chromium-free coated steel sheet.
There have been proposed various techniques relating to the chromium-free coated steel sheet. Examples of the techniques include: a technique utilizing a passivation effect of molybdenum acid and tungsten acid that belong to the same Group IVA as chromium acid; a technique of employing a metallic salt of transitional metal such as Ti, Zr, V, Mn, Ni, Co or of rare earth element such as La, Ce; a technique of using, as a base, a chelating agent such as polyvalent phenolic carboxylic acid like tannic acid or a compound including S and N; a technique of forming a polysiloxane coating film using a silane coupling agent; and a technique as a combination of these techniques.
Specific examples of those techniques are as follows:                (1) A technique of forming a coating film from a treatment solution prepared by blending: a coating agent obtained by reacting an organic resin such as polyvinyl phenol derivatives with an acid component and an epoxy compound; a silane coupling agent; a vanadium compound; and the like (see, for example, JP 2003-013252 A, JP 2001-181860 A, JP 2004-263252 A, and JP 2003-155452 A).        (2) A technique of forming a coating film including a water-soluble resin, a thiocarbonyl group, a vanadate compound, and a phosphoric acid (see, for example, JP 3549455 B).        (3) A technique of forming a coating film using a treatment solution containing a metallic (such as Ti) compound, fluorides, and inorganic acid such as a phosphate compounds and organic acid (see JP 3302677 B, JP 2002-105658 A, JP 2004-183015 A, JP 2003-171778 A, JP 2001-271175 A, JP 2006-213958 A and JP 2005-048199 A).        (4) A technique of forming a composite coating film from rare earth elements such as Ce, La, Y, and Ti, Zr elements and then forming by concentration an oxide layer on a coating interface side and a hydroxide layer on a surface side in the coating film (JP 2001-234358 A), a technique of forming a composite coating film of Ce and Si oxide (JP 3596665 B).        (5) A technique of forming, as an under layer, a phosphate and/or a phosphate compound coating film containing oxide and forming, as an upper layer thereof, an organic composite coating film formed of a resin coating film (see, for example, JP 2002-053980 A and JP 2002-053979 A).        (6) A technique of forming a composite coating film containing a specific inhibitor component and a silica/zirconium compounds (see, for example, JP 2008-169470 A).        (7) A technique of forming a composite coating film containing: a water-soluble zirconium compound; a tetraalkoxysilane; a compound having an epoxy group; a chelating agent; a vanadic acid; and a predetermined metallic compound (JP 2010-255105 A).        
The films formed by the above-mentioned techniques are supposed to suppress occurrence of white rust in zinc through combined addition of organic components or inorganic components. For example, according to the techniques of the above-mentioned references (1) and (2), corrosion resistance is ensured by adding, in principle, an organic resin. However, a coating film thus formed of an organic resin fails to exhibit satisfactory continuity. Further, welding dissolves the organic compound, and thus weldability cannot be ensured.
The techniques of the above-mentioned references (3) and (4) propose an inorganic-only film that is completely free of any organic component. However, such a composite film formed by metal oxide and metal hydroxide must be made thick to attain sufficient corrosion resistance as a zinc or zinc alloy coated steel sheet. Further, the techniques of the references (3) and (4) cannot satisfy corrosion resistance as well as conductivity and weldability in a compatible manner because a zinc or zinc alloy coated steel sheet surface thereof is covered with a nonconductive film (insulating film) such as zinc phosphate, which is disadvantageous in terms of attaining excellent conductivity and weldability, as in the case of the techniques of the above-mentioned references (1) and (2).
The technique of the above-mentioned reference (5) is focused on a fact that the conductivity of a surface of a surface-treated steel sheet depends on film thickness of an insulating film covering the surface of the steel sheet, and makes it possible to obtain excellent conductivity by reducing the thickness of the insulating coating film. To reduce the film thickness of the insulating coating film is also preferred in terms of ensuring weldability. However, when the film thickness is reduced, the corrosion resistance of the steel sheet is degraded, thereby making it difficult to obtain a surface-treated steel sheet which is good in all of corrosion resistance, conductivity, and weldability.
The technique of the above-mentioned reference (6) utilizes, as an inhibitor component, a passivation effect of a vanadate compound and a low soluble metallic salt derived from a phosphate compound and forms, as a skeleton of the film, a composite coating film of a zirconium compound, silica particles, and a silane coupling agent, to thereby manifest excellent corrosion resistance. However, to ensure conductivity at higher level as has been conventionally attained through a chromate treatment, the film thickness needs to be reduced, which makes it difficult to satisfy corrosion resistance as well as conductivity and weldability in a compatible manner.
The technique disclosed in the above-mentioned reference (7) is capable of providing a zinc or zinc alloy coated steel sheet having corrosion resistance and adhesion properties, and is excellent in continuity under low contact pressure and corrosion resistance. However, it was found that good weldability could not be attained even with the use of this surface-treated steel sheet.
As described above, in the conventional chromium-free coated steel sheet hitherto proposed, the film thickness of a highly-insulating coating film needs to be increased to reliably obtain as good corrosion resistance as a conventional chromate coating. This inevitably makes it difficult for the conventional chromium-free coated steel sheet to ensure good conductivity and weldability, and therefore, the conventional chromium-free coated steel sheet is hardly satisfactory to be introduced as an alternative technique to chromate treatment. Further, as described above, it is necessary to design the coating film similarly to the conventional chromate coating to ensure the weldability. However, none of the above-mentioned techniques gives any consideration to ensuring good weldability in such a circumstance as described above.
It could therefore be helpful to provide a zinc or zinc alloy coated steel sheet that is not only excellent in corrosion resistance and top coating properties, but also capable of attaining corrosion resistance, continuity, and weldability in a balanced manner without containing chromium compound. It could also be helpful to provide a method of manufacturing the zinc or zinc alloy coated steel sheet and a surface-treatment solution therefor.